Bellows are used in a variety of applications such as pressure and motion sensing, sealing, surge arresting, mechanical and pressure actuation, flexible reservoirs, pumps, fluid isolators, etc. In most applications, the motion of the bellows is relatively slow, that is, the plurality of "folds" of the bellows, commonly referred to as "convolutions," expand or contract at generally the same rate. There are applications, however, where the motion of one end of the bellows relative to the other end of the bellows can be so rapid that the convolutions near the moving end begin to close while the convolutions adjacent the stationary end remain relatively motionless. Such a condition can occur because bellows are characteristically flexible devices, and the individual convolutions have sufficient mass causing them to resist rapid motion. When the moving termination of the bellows travels at a high rate of speed, the inertial resistance of the individual convolutions to such motion results in the progressive collapsing of the convolutions to a solid height beginning with the convolutions nearest the moving termination. Once a convolution collapses to a solid height, the convolution is accelerated to the velocity of the moving termination. This effect can be visualized through a common child's toy--a helical, highly flexial, flat, circular spring, commonly known as a "Slinky." If this toy is placed on a smooth surface with its coils spaced slightly open and then one end thereof is swept rapidly by one's hand toward the other end, the coils can be seen and heard stacking against the rapidly moving end while the coils at the opposite end are still stationary. This effect can be readily seen since the spring rate of this toy is extremely low so that even the relatively slow motion of one's hand can induce the aforementioned effect.
In certain bellows applications, the motion of the associated actuating mechanism that compresses the bellows can be extremely rapid. For example, during an electrical power disconnect, a high voltage puffer-type switch that is used to make and break electrical connections can typically move a distance of four inches within a period of 12 milliseconds. This type of switch, which can interrupt power service voltages up to 500,000 volts, utilizes an inert gas, sulfur hexafluoride or SF6, which is contained in the switch under pressure to "blow out" the electrical arc that is formed when the electrical contacts separate. In order to achieve arc extinguishment, the switch contacts are in the form of a piston in a closely fitting cylinder. As one contact, the piston, is rapidly separated from the other contact, the cylinder, a vacuum is formed inside the cylinder. As the edge of the piston passes the edge of the cylinder, SF6 rushes into the vacuum keeping the arc extinguished until the two contacts are sufficiently apart, thus preventing arcing. The SF6 is indispensable to achieve an arc-less disconnection. Without the SF6, an arc would bridge the contacts and the energy released would cause the switch to detonate, presenting a significant danger to workers and causing a power outage over a wide geographic area.
Developers of high voltage switchgear must achieve positive, long-term containment of SF6. To transmit mechanical motion into the sealed, pressurized chamber, a metallic bellows is used to join the linear motion actuating rod to the pressurized chamber. A metallic bellows, rather than a sliding seal, is utilized because of the impermeability of a metallic bellows and its relative insensitivity to extreme outdoor temperatures and atmospheric gases, such as ozone.
Metallic bellows provide a seal with a sufficiently low leak rate permitting a typical high voltage switch to remain in outdoor service for fifteen to twenty years. Historically, bellows manufacturers have experienced difficulty in providing more than 1000-2000 switch actuations before bellows failure. Such a failure rate was acceptable in the past, but changes in the power industry require longer and more reliable switch life, up to 40 years. The key issues affecting switching requirements and the use of SF6 to prevent switch arcing are:
1) The sale of electric power over long distances requires more frequent switching to redirect power to the end user; PA1 2) SF6 is a powerful greenhouse gas that must be totally contained; and PA1 3) The increasing competitiveness of the electric power industry requires that the switchgear have an operating life of three to four decades. PA1 1. The wide ambient temperature range to which switchgear is subjected would result in wide variations in damping; PA1 2. The existing space constraints of switchgear makes the addition of a viscous damping system impossible; PA1 3. A liquid or semisolid material cannot be used within the switchgear because of possible contact contamination; and PA1 4. The cost of a viscous damping system is several times the cost of the bellows seal, making it impractical. PA1 1. Applying a direct drag against the bellows welds to a degree sufficient to dampen the levels of energy required induces severe wear on the weld beads and causes premature bellows failure; PA1 2. Introducing additional frictional elements within the bellows to apply "drag" against other components outside the bellows violates space constraints; and PA1 3. The cost of additional frictional elements to absorb the level of energy involved makes this approach impractical.
SF6 is a primary factor affecting bellows integrity. The use of this gas has come under close scrutiny by the Environmental Protection Agency because, while it is the least used volumetrically of the greenhouse gasses, it causes the most severe effects, as shown in Table 1 below.
TABLE 1 ______________________________________ GREENHOUSE GASES AVERAGE RESIDENCE GLOBAL IN TROPOSPHERE, WARMING GAS YEARS POTENTIAL, GWP ______________________________________ Carbon dioxide &lt;10 1 Methane, CH4 10 25 Nitrous oxides, N2Ox 170 230 Chlorofluorocarbons, 60-100 15,000 CFCs Sulfur hexafluoride, 3,200 16,500 SF6 ______________________________________
In addition to the atmospheric effects of SF6, or perhaps as a result of it, this gas is also subject to wide price fluctuations. From November 1994 to November 1996, the cost of SF6 increased by more than 500%, adding to the demand for efficient sealing and long service life. For these reasons, it has become necessary to develop a method of dramatically increasing the life of the bellows used to seal chambers containing SF6 gas, or the like.
To determine the behavior of metallic bellows during switchgear actuation, the motion of the bellows in a cutaway switchgear was studied using high speed video imaging. This analysis revealed the behavior of bellows when being compressed at a high linear velocity. When a high voltage puffer-type switch disconnects an electrical circuit, the seal bellows undergoes compression at velocities over 27 ft/sec. The bellows behavior is shown in FIGS. 1 and 2 and described as follows:
1. An actuation mechanism is utilized to deliver the energy stored within a large, preloaded spring to an actuating rod which passes through the housing of the switch to the movable contact therein. At the point where the rod passes into the switch housing, a seal bellows is positioned concentrically over the outside of the rod such that one end of the bellows is attached to the rod by means of a movable termination and the other end of the bellows is attached to the switch housing through a stationary termination.
2. As the actuation mechanism is tripped, releasing the preloaded spring, the actuating rod and its associated mechanism moves toward the left at a high rate of speed causing the electrical contacts to open and causing the bellows to collapse toward its solid height. At the end of the stroke of the actuating rod, its associated mechanism hits a mechanical stop, often configured as a hard rubber bumper, causing the mechanism and the rod to rebound off the stop.
3. The motion of a conventional bellows during this aforementioned action is not readily observable by the unaided eye. Without the aid of high-speed video imaging, the bellows simply appears to be translated from its extended length to its collapsed length, a distance of about four inches, in 1/60 of a second or less.
The motion of the bellows as observed with high-speed video imaging is complex. Visually, the convolutions within the bellows appear to be swept up by the moving termination and thrown against the stationary termination where they rebound toward the moving termination. There is a brief moment when the convolutions are traveling toward the stationary termination and the moving termination is rebounding in the opposite direction. The result is that the convolutions nearest the moving terminations, i.e., the convolutions, that begin moving first, are stretched open to a width of up to double their normal free width. This stretching motion induces stresses in the affected convolutions of over twice the stress that the convolutions would experience in slow-speed, isometric motion. The resulting stress is greatest within the convolution adjacent the movable termination. At the stationary end, the almost solid mass of the fully collapsed bellows impacts the stationary termination. The stationary termination is solidly attached to the body of the switch housing, giving the system a high coefficient of restitution. Nearly all of the bellows convolutions impact the stationary termination as a solid mass in their closed, solid height condition, with the exception of a few convolutions that are near the moving termination and are being stretched open a distance substantially greater than their free width. All of the energy of the moving mass is delivered to the last convolution which is welded to the stationary termination.
In a typical electrical switchgear seal, a welded bellows may have an outside diameter of 1.37 in., a length of 4.50 in., and be fabricated from stainless steel strip material having a typical thickness of 0.006 in. A bellows of this size might weigh 0.375 lb. In operation, the bellows could travel a distance of 2 in. in extension from its free length to 2 in. in compression from its free length for a total distance of 4 in. At a typical velocity of 27.7 ft/sec, the kinetic energy delivered to the last convolution within this bellows is: EQU E=1/2MV.sup.2 =Wv.sup.2 /2g=4.5 ft-lbs
This kinetic energy is delivered in less than a millisecond to the stationary termination through the last convolution adjoining it. The convolution, having a thickness of roughly two sheets of paper, is severely strained, and the resulting stress is concentrated at the interior diameter and exterior diameter welds of same. With a system coefficient of restitution of perhaps 0.8, 80% of the delivered energy is returned to the mass of the bellows as it rebounds away from the stationary termination. The bellows continues to act as an almost solid, compressed mass as it rebounds, but it is not solid since its structure is similar to that of a spring. After the rebound, the mass of closed convolutions travels back toward the moving termination. As the center of the bellows mass leaves the stationary termination, the convolutions closest to that termination are pulled open to roughly twice their free pitch, the most severe opening occurring at the convolution adjoining the stationary termination, i.e., the convolution that just received the 4.5 ft-lbs of kinetic energy. The convolutions adjacent the convolution adjoining the stationary termination are pulled open to a lesser degree nearer the moving mass of the bellows causing an oscillation that repeats for five or six diminishing cycles as the kinetic energy of the bellows is released in the form of internal heating.
It is apparent from the foregoing, that under the effects of high velocity, the convolutions of a bellows do not open and close isometrically as they would at low velocity. A small number of convolutions, roughly 10% of the convolutions located near the movable and stationary terminations, are subject to most of the flexing while the remaining 90% of the convolutions remain relatively closed and in a condition of rebounding from one termination to the other. It is also apparent that in a single switch closure, a small number of convolutions undergo the equivalent of five or six switch closures. Therefore, motion of the convolutions within a conventional bellows is not a one-for-one relationship with switch operation, but a five-to-one relationship for certain convolutions depending upon their location within the bellows.
In terms of vibration, the above described system is clearly an underdamped system. The bellows dissipates its energy sinusoidally within five or six cycles. The first and most obvious approach to remedy the problem of underdamping is to introduce damping in the form of a viscous system or through friction on the convolutions. Both approaches have been used to correct vibration-induced, premature bellows failures. Viscous damping involves pumping a fluid through a small orifice located in a termination positioned midway in the bellows and has been done successfully by NASA for a bellows used on a reaction wheel vibration isolation system on the Hubble Space Telescope. Coulomb damping by applying frictional drag to the edges of the outer diameter or inner diameter of the bellows has been used for many years. This latter approach is used where bellows are often employed as chemical pump rotary shaft seals. Bellows oscillations induced by pump vibration are dampened by a ribbon-type spring installed within the bellows housing. The repeated bends in the spring cause it to apply a load to weld beads on the outer diameter of the welded bellows. The frictional drag on the weld beads prevents the bellows from oscillating which would fatigue the bellows or lower the face load on the seal.
Viscous damping is impractical in switchgear applications for several reasons:
Coulomb damping is a reasonable alternative for small vibrations, but is not suitable for long-stroke, high velocity motion for the following reasons:
Another approach to overcoming the problems resulting from an underdamped system is to fabricate the bellows from different material thicknesses for the convolutions with the objective of creating bellows sections having a higher spring rate at each end of the bellows. It was thought that these higher spring rate sections would open up the nearly solid mass of closed convolutions by reflecting the kinetic energy back through itself in several smaller waves which were out of phase with respect to one another. A bellows was fabricated where approximately 12% of the total convolutions were made from material 29% thicker than the regular convolutions. These "heavier" convolutions were evenly distributed with 6% at each end of the bellows, and were welded to the movable and stationary terminations. The effect of this configuration increased bellows life by over 300%. High speed imaging disclosed how this occurred.
Tests of this latter bellows structure revealed that when the movable termination began compressing the bellows, the thicker convolutions adjoining the movable termination resisted the complete closure experienced by the regular bellows. The more gradual closure of the convolutions transferred some, but not all, of the kinetic energy to the adjoining regular convolutions and the transfer was not immediate resulting in a more gradual, progressive closure of the convolutions comprising the bellows. Ultimately, however, the entire bellows, with the exception of the thicker convolutions adjoining the stationary termination, became a solid mass and moved toward the stationary termination, compressing the thicker convolutions. At this point, the thicker convolutions, while being compressed, reflected a portion of their kinetic energy back through the nearly solid bellows mass so that a first wave was seen passing through it toward the movable termination. When the thicker convolutions became completely compressed, the convolutions reflected a second wave from the stationary termination. This second wave was out of phase with the first wave and traveled through the bellows behind the first wave. The end result was that the bellows was no longer a solid mass, but was broken into three regions with traveling waves acting as boundaries of the regions. The thicker convolutions near the stationary termination began to reopen, moving the divided bellows back toward the moving termination.
When the first wave encountered the thicker convolutions at the moving termination, a portion of energy within the wave was reflected back into the waves approaching it. The colliding waves each reflected a portion of their respective energy in opposite directions, and this reoccurred as each wave rebounded and collided with oncoming waves. The mass of the nearly solid bellows traveled only once toward the stationary end at the beginning of the operation, but then its motion resolved into a flutter-like effect throughout the entire bellows as each wave intersected another wave. These tests confirmed that the use of different spring rates strategically placed in the bellows could dramatically increase bellows life.